Histologic and ultrastructural studies were made of the changes induced in cardiac and skeletal muscle of Calomys callosus, a small rodent, by infection with T. cruzi. Macrophages appear to be the main type of immune effector cell involved in the inflammatory process. THe myocardial damage was rapidly followed by interstitial fibrosis, and this fibrosis showed significant regression at 2 months after infection. These features make C. callosus a valuable model for the study of T. cruzi myocarditis.